Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::needless_borrowed_reference: this pattern takes a reference on something that is being dereferenced

What it meansChecks for bindings that needlessly destructure a reference and borrow the inner value with &ref. It is in the complexity group and warns by default.

What you see

warning: this pattern takes a reference on something that is being dereferenced
 --> main.rs:4:22
  |
4 | v.iter_mut().filter(|&ref a| a.is_empty());
  |                      ^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#needless_borrowed_reference
  = note: `#[warn(clippy::needless_borrowed_reference)]` on by default
help: try removing the `&ref` part
  |
4 - v.iter_mut().filter(|&ref a| a.is_empty());
4 + v.iter_mut().filter(|a| a.is_empty());
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

This pattern has no effect in almost all cases.

Code that does something simple in a complicated way. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let mut v = Vec::<String>::new();
v.iter_mut().filter(|&ref a| a.is_empty());

if let &[ref first, ref second] = v.as_slice() {}

Write this instead

let mut v = Vec::<String>::new();
v.iter_mut().filter(|a| a.is_empty());

if let [first, second] = v.as_slice() {}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::needless_borrowed_reference)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
needless_borrowed_reference = "allow"

One run

cargo clippy -- -A clippy::needless-borrowed-reference

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for bindings that needlessly destructure a reference and borrow the inner value with &ref.

Why is this bad?

This pattern has no effect in almost all cases.

Example

let mut v = Vec::<String>::new();
v.iter_mut().filter(|&ref a| a.is_empty());

if let &[ref first, ref second] = v.as_slice() {}

Use instead:

let mut v = Vec::<String>::new();
v.iter_mut().filter(|a| a.is_empty());

if let [first, second] = v.as_slice() {}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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